Colorado Monthly Precipitation Summary

Colorado Monthly Precipitation Summary

Since the first of May, mountain weather conditions in Colorado have been dominated by a drier weather pattern with snowpack melting earlier than normal. Statewide snowpack on the first of June was 24 percent of normal. “While these conditions are more favorable for hiking Colorado’s mountains, snowpack and precipitation shortages in May further depleted Colorado’s summer water supply outlook,” says Brian Domonkos, Colorado Snow Survey Supervisor, Hydrologist with the NRCS. In a normal year, on the first of June there are 21 of a total 115 SNOw TELemetry network sites in the mountains of Colorado that show an inch or more of snow water equivalent. However, this year on the first of June only ten sites had any measurable snow at all. With snowpack on its way out, other contributors serve as indicators for summer water supply predictions such as precipitation, reservoirs, and current streamflows.

At 55 percent of average, May 2018 precipitation was the second driest month of this water year behind December which was 50 percent of normal. Unlike April, when precipitation was near 95 percent of normal in the northern half of Colorado, May precipitation failed to top 75 percent of normal in any of the major basins across the state. Only three of the major watersheds in Colorado received at least half of their average monthly precipitation; the South Platte, Colorado, and combined Southwestern basins. Fortunately, statewide year-to-date precipitation showed only a small decrease from 72 percent of average on the first of May, to now 70 percent of normal on the first of June.

Statewide reservoirs remain slightly above normal, at 106 percent of average, dropping six percent since May 1. Most of the declines occurred in the southern basins including the Gunnison and combined southwest basins where storage fell 14 and 16 percent, respectively, and most notably in the Rio Grande basin, which fell 25 percent. These decreases in reservoir storage are due to calls for irrigation to supplement receding runoff. Many streams which typically peak in June, peaked in May this year, two to three weeks early. In dry years especially, Domonkos adds, “When streamflows peak early, less water is available later in the summer when it is needed most.” With peak streamflow in the past, streamflows are forecasted near record lows at some stream gauges in the Gunnison, through the southwest corner of the state to the Rio Grande. Shifting the focus northward, basin-wide reservoir storage remains above normal, above 110 percent. Streamflow forecasts however are generally lower, between 50 and 85 percent of normal.